In the current market, a display screen quite differs from a mobile phone screen, luminous range of the display screen also differs from luminous range of the mobile phone, and display effect of a screen is influenced greatly by environment. The present optical elements are mostly current-driven and sense luminous intensity by matching an appropriate load resistor. Disadvantage of the present optical elements is: it is difficult to determine resistance of the load resistor in the case that there is a big change in range of luminous intensity. Even the resistance of the load resistor is determined, since the resistance is a constant value, therefore there is a small adaptability range of the resistance on luminous intensity.